I want to double the recipe, substituting a cup of chopped okra for the additional 1/2 cup each of celery and bell pepper otherwise called for. What concerns me about doing this is okra's mucilaginousness; I know that my great-grandmother adjusted her étouffée recipe when adding okra to it to prevent it from becoming too gummy but can't remember how.

The gumminess in okra is from something that is inside the pods. If you add the pods whole, they won't be gummy. You can adjust how much you cut to figure out the ideal texture -- try cutting some and not others.